1. Field of the Invention
The present invention is directed to a regulated inverter of the type having a switched output stage, as well as to a driver circuit therefor.
2. Description of the Prior Art
A drive arrangement of the above type suited for all switch output stages of regulated inverse rectifiers (inverters) in half-bridge or full-bridge circuitry, especially for inverse rectifiers wherein high powers must be regulated with extreme exactitude. This is primarily the case with gradient amplifiers in nuclear magnetic resonance tomography devices, however, such a drive arrangement is also applicable to inductive heating means in X-ray devices or for drive control of electromotors.
In the example of utilization in a gradient amplifier, with current on the order of magnitude of 300 A an alternating voltage is created on the order of magnitude of .+-.300 V by means of a bridge circuit. The amplifier must have such a high precision that the current conduction in the mA-range is adjustable. Therefore, the on-phases of the individual switching elements in the bridge circuit basically must be continuously varied as to their respective durations. For this reason, driving circuits of gradient amplifiers heretofore have been purely analog in their structure and operation, because an arbitrarily fine control of the switching time of the switching transistor is thereby permitted. A digital, purely clock-driven circuit is not realistically realizable by today's means, because clock frequencies in the GHz-range would be necessary in order to obtain the required fine time resolution.
Furthermore it is necessary for the driving circuit to insert dead time between the on-phases of the power transistors in a bridge-arm to avoid what is known as a bridge short circuit. Such dead times have been created in known drive circuits by an asynchronous monostable flip-flop in connection with locking logic. Such a circuit is shown in FIG. 2a of the article "Reliable Operation of MOSFETs in Bridge Circuits" by Hans R. Hassig and Patrick Zoller in the journal Elektronik, Volume 10/12 May 1989, pages 55-63.
This simple circuit, however, is not suitable for the much more critical application of a gradient amplifier. There, an analog circuit with a high components cost must be used. Besides, a costly adjustment of the monostable flip-flop is required.
German OS 43 00 981 discloses a drive of the general type described initially. A microcontroller is designed as a pulse-width modulator and creates drive signals (pulse sequences) in cooperation with a drive logic, each of whose on-phases are separated by a dead time. Since the microcontroller is clock-driven, the leading as well as the trailing edges of the pulses of the drive signals are created at fixed times.
The drive arrangement of German OS 43 00 981 disclosed for use for driving an inverse rectifier for a motor or transducer. This known circuit, however, is not suitable for very high-precision demands such as exist in gradient amplifiers. As already discussed, the clock frequency of the microcontroller would have to be in the GHz-range in order to achieve the necessary time resolution.
Likewise, a drive logic circuit is known from Japanese Application 5-276 792 which creates the leading as well as the trailing edges of drive signals at fixed times.